Broccoli and diabetes

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Behind the Headlines

Thursday August 7 2008

Eating broccoli should not be considered to counteract diabetes

“Eating broccoli could reverse the damage caused by diabetes to heart blood vessels”, BBC News reported. It said that researchers have found that the compound sulforaphane, found in the vegetable, encourages the production of enzymes which protect blood vessels and cause a reduction in the number of molecules that can damage cells.

This story is based on a complex laboratory study in which sulforaphane was directly applied to blood vessels that had been damaged by high blood sugar levels. It found that the compound reduced the production of potentially damaging molecules called reactive oxygen species. However, the results have been overinterpreted by the news; applying the compound in broccoli to cells in the laboratory is not comparable to eating broccoli. The blood vessel cells were not taken from a person with diabetes but had been incubated with sugar. It is unclear what effects sulforaphane would have on the blood vessels of a person with diabetes, and whether it would protect them from damage or have any effect upon the disease process. Optimal blood sugar control through diet and medication remains the best option for people with diabetes.

Where did the story come from?

Dr Mingzhan Xue and colleagues from the University of Warwick and University of Essex carried out the research. It was supported by Juvenile Diabetes Research Foundation International, the Wellcome Trust, and the Biotechnology and Biosciences Research Council. The study was published in the peer-reviewed medical journal: Diabetes.

What kind of scientific study was this?

The aim of this laboratory study was to look at whether sulforaphane, a compound found in broccoli, could prevent metabolic damage to small blood vessels caused by high blood sugar. Sulforaphane activates a protein called nrf2, which initiates the production of a number of enzymes that protect cells from potentially damaging chemicals, including a type of free radical called reactive oxygen species (ROS).

The researchers incubated cells taken from the lining of human small blood vessels with two different concentrations of sugar – low and high. They then used laboratory methods to see what effects incubation with sulforaphane had on a range of complex metabolic and biochemical pathways. The researchers used concentrations of sulforaphane that they said were representative of the levels that have been reported to be found in the blood stream after eating broccoli.

What were the results of the study?

The researchers found that activation of the protein nrf2 by sulforaphane, caused increased expression of various protective and metabolic enzymes, including three to five fold increases in the enzymes transketolase and glutathione reductase.

Incubating blood vessel cells in high sugar concentrations resulted in a three-fold increase of the potentially harmful free radical ROS, but adding sulforaphane reduced ROS levels by 73%. The enzyme transketolase played a role in this reduction. Sulforaphane also prevented the production of other chemicals that may potentially cause blood cell dysfunction in high blood sugar conditions.

What interpretations did the researchers draw from these results?

The researchers concluded that activation of nrf2 may prevent the biochemical dysfunction of cells that line the inside of blood vessels caused by high levels of blood sugar.

What does the NHS Knowledge Service make of this study?

The news report overinterpreted the results of this complex laboratory study.

Although the researchers say that the concentrations of sulforaphane used were limited to those ‘found in plasma after consumption of broccoli’ it is unclear how these laboratory effects would be comparable to the real life situation of eating broccoli or what frequency or quantity of consumption of broccoli would be required to mimic these effects.

The blood vessel cells were not taken from a person with diabetes, but had instead been incubated with sugar. Short-term incubation of cells in high sugar concentrations cannot be directly related to the situation in a person with diabetes.

It is unclear whether any of the biochemical and metabolic changes seen in the blood vessels would relate to functional change in the blood vessels of a person with diabetes and whether it would protect them from damage or have any effect upon the disease process.

The news article suggests that ‘eating broccoli could reverse damage caused by diabetes to heart blood vessels’. It is unclear from the journal article where in the body the blood vessel cells were taken from, but they were from small blood vessels – microvessels. Although poor blood sugar control can cause significant small vessel damage in the body (for example to the retina, kidney and nerve cells), heart disease as a complication of diabetes is considered to be a large vessel – macrovascular – complication.

There is no indication from this study how the biochemical changes could reverse damage that had already been caused.

Optimal blood sugar control through diet and medication remains the best option for people with diabetes, and broccoli should be considered only as part of a healthy diet.